The present invention relates to a method and apparatus for producing alcohol by fermentation in general, and more particularly to such a method and apparatus as used in producing the alcohol on a continuous basis.
It is well known that alcohol can be obtained by fermentation of a mash containing starch and/or sugar which constitute nutrients for zymogenic organisms, especially yeasts. In recent years, processes and apparatus have found an ever increasing use in which the fermentation takes place on a continuous basis, rather than in batches, that is, in which the mash is supplied at a steady rate into the apparatus and the fermented mash is withdrawn from a different part of the apparatus at the same rate. Usually, such conventional apparatus includes a plurality of fermentation vessels which are connected in series and between which the mash being fermented is being successively transferred. The continuous production reduces the amount of time spent in the production of alcohol, at least by the amount of time spent in the batch process for filling and emptying the fermentation vessels. However, it has been found that the continuous fermentation cannot be continued indefinitely, particularly since the mash in the vessels eventually becomes "poisoned" by microorganisms which cause chemical reactions different from alcohol fermentation, such as acid fermentation. Therefore, it is necessary from time to time to empty the fermentation vessels and to clean the same, and then to restart the alcohol fermentation process by introducing fresh yeast which does not contain the undesirable microorganisms, or contains only a negligible proportion thereof.
Of course, if it were attempted to clean all the fermentation vessels at the same time, a substantial amount of time would be lost since the cleaning operation would have to await the emptying of the first fermentation vessel in the series, and would last beyond the emptying of the last fermentation vessel in the series, all of which time would be lost as far as the fermentation operation is concerned. During this time, the supply of the mash to be fermented to the apparatus would have to be interrupted, and if the mash were continuously made, as is usually the case, it would have to be accumulated in storage vessels in the interim. Therefore, it is advantageous to maintain a steady flow of the mash into and of the fermented product out of the apparatus, particularly since the accumulation of the mash in the storage vessel brings about at least one significant problem, that is, the contamination of the mash from the ambient atmosphere with the undesirable microorganisms or other contaminants. Obviously, such a contamination results in a loss in the yield of alcohol.
This problem could be at least partially avoided by the provision of an additional fermentation vessel in the series of such vessels, which additional vessel would be put into operation while one or another of the other fermentation vessels is being cleaned. However, under these circumstances, each of the fermentation vessels would have to be capable of serving as the first, second, third, fourth etc. vessel in the series. A continuous alcohol fermentation process and apparatus of this type is known from the published German patent application DE-OS No. 1,642,693, wherein the fermentation vessels of the augmented series can be individually taken out of operation for emptying, cleaning and disinfection by effecting changes in the manner in which the fermentation vessels are interconnected. Thus, the continuous flow of the mash into and of the fermented product out of the apparatus need not be interrupted. While at least theoretically this arrangement is capable of operating in a continuous manner, the piping diagram, the expenditure for the various switching valves, and the operation of the apparatus are so intricate and complex that the total cost is almost prohibitive. Possibly for this reason, arrangements of this type did not find widespread acceptance in the alcohol-producing industry.